The kinetics of the N-demethylation of meperidine and methadone, hydroxylation of hexobarbital and conjugation of morphine with glucuronic acid have been studied in hepatic microsomes obtained from fetal stumptailed monkeys (Macaca arctoides) during the last half of gestation. For each substrate the apparent Km remained constant during the time period examined while Vmax/mg microsomal protein increased. In the case of meperidine Vmax was linearly related to fetal body weight or crown-rump length whether expressed per mg microsomal protein or per nmole cytochrome P-450. For methadone, Vmax was linearly related to fetal body weight or crown-rump length only when expressed per mg microsomal protein. No significant correlation between methadone Vmax and fetal body weight or crown-rump length was observed when Vmax was expressed per nmole cytochrome P-450. The pharmacokinetics of antipyrine and hexobarbital were also studied in pregnant and nonpregnant monkeys with each animal serving as its own nonpregnant control. No significant differences were observed in half-life, apparent volume of distribution or plasma clearance of these drugs during pregnancy or at 8 weeks and 6 months postpartum.